The primary goal of the project is to determine the behavioral, ecological and reproductive factors which characterize a population of house mice (Mus musculus) at each of several successive stages of crowding. The rate of population growth was regulated; after the adult population reached 100, the population was allowed to double approximately every 200 days. After 1200 days the population reached about 1600 and then began to decline because of the death rate exceeded the rate of recruitment. The rate of reproduction began to further decline with no survival of pups beyond three days of age, and the population size decreased to about 1200. Removal of all but the youngest generation of animals, which reduced population size to less than 500, a size at which a high rate of reproduction had occurred earlier, did not stimulate reproduction and the population continued to decrease until the study was terminated. The tendency to remain in highly crowded groups even after the population size was reduced was the primary cause of the collapse of reproduction because the mice exhibited poor and abnormal parental behavior under highly crowded conditions.